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Subject[PATCH 0/7][v8] Container-init signal semantics

Patch 5/7 is new in this set and fixes a bug. Remaining patches are
just a forward-port from previous version and I believe they address
all comments I have received.

Oleg please sign-off/ack if you agree.

---
Container-init must behave like global-init to processes within the
container and hence it must be immune to unhandled fatal signals from
within the container (i.e SIG_DFL signals that terminate the process).

But the same container-init must behave like a normal process to
processes in ancestor namespaces and so if it receives the same fatal
signal from a process in ancestor namespace, the signal must be
processed.

Implementing these semantics requires that send_signal() determine pid
namespace of the sender but since signals can originate from workqueues/
interrupt-handlers, determining pid namespace of sender may not always
be possible or safe.

This patchset implements the design/simplified semantics suggested by
Oleg Nesterov. The simplified semantics for container-init are:

- container-init must never be terminated by a signal from a
descendant process.

- container-init must never be immune to SIGKILL from an ancestor
namespace (so a process in parent namespace must always be able
to terminate a descendant container).

- container-init may be immune to unhandled fatal signals (like
SIGUSR1) even if they are from ancestor namespace. SIGKILL/SIGSTOP
are the only reliable signals to a container-init from ancestor
namespace.

Patches in this set:

[PATCH 1/7] Remove 'handler' parameter to tracehook functions
[PATCH 2/7] Protect init from unwanted signals more
[PATCH 3/7] Add from_ancestor_ns parameter to send_signal()
[PATCH 4/7] Protect cinit from unblocked SIG_DFL signals
[PATCH 5/7] zap_pid_ns_process() should use force_sig()
[PATCH 6/7] Protect cinit from blocked fatal signals
[PATCH 7/7] SI_USER: Masquerade si_pid when crossing pid ns boundary

Changelog[v8]:

- Bugfix (new patch, 5/7): Nested container-init not terminated when
parent container-init exits and calls zap_pid_ns_processes().
- Dropped old patch 7/7 which showed SIG_DFL signals to init as
"ignored" in /proc (we were undecided on whether its good or bad).

Changelog[v7]:
- siginfo_from_user() and siginfo_from_ancestor_ns() are fairly simple
and used only in send_signal(). Remove them and move the logic into
send_signal() (Patch 4/7)

- Update /proc/pid/status to include SIG_DFL signals to init in the
"ignored" set (and remove the TODO in Patch 0/7) (Patch 7/7)

Changelog[v6]:

- Patches 3,4: Have kill_pid_info_as_uid() pass in 'from_ancestor_ns'
parameter to __send_signal() and remove SI_ASYNCIO check in
siginfo_from_user().
- Patches 4,6: Update changelog and simplify code

Changelog[v5]:
- Patch 2/6: Remove SIG_IGN check in sig_task_ignored() and let
sig_handler_ignored() check SIG_IGN.
- Patch 3/6. Put siginfo_from_ancestor_ns() back under CONFIG_PID_NS
and remove warning in rt_sigqueueinfo().
- (Patch 5/6)Simplify check in get_signal_to_deliver()
- (Patch 6/6)Simplify masquerading pid
- LTP-20081219-intermediate showed no new errors on 2.6.28-rc5-mm2.

Changelog[v4]:
- [Bugfix] Patch 3/7. Check ns == NULL in siginfo_from_ancestor_ns().
Although http://lkml.org/lkml/2008/12/16/502 makes it less likely
that ns == NULL, looks like an explicit check won't hurt ?
- Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify logic as
suggested by Oleg Nesterov.
- Dropped patch that set SIGNAL_UNKILLABLE_FROM_NS and set
SIGNAL_UNKILLABLE in patch 5/7 to be bisect-safe.
- Add a warning in rt_sigqueueinfo() if SI_ASYNCIO is used
(patch 3/7)
- Added two patches (6/7 and 7/7) to masquerade si_pid for
SI_USER and SI_TKILL


Changelog[v3]:
Changes based on discussions of previous version:
http://lkml.org/lkml/2008/11/25/458

Major changes:

- Define SIGNAL_UNKILLABLE_FROM_NS and use in container-inits to
skip fatal signals from same namespace but process SIGKILL/SIGSTOP
from ancestor namespace.
- Use SI_FROMUSER() and si_code != SI_ASYNCIO to determine if
it is safe to dereference pid-namespace of caller. Highly
experimental :-)
- Masquerading si_pid when crossing namespace boundary: relevant
patches merged in -mm and dropped from this set.

Minor changes:

- Remove 'handler' parameter to tracehook functions
- Update sig_ignored() to drop SIG_DFL signals to global init early
(tried to address Roland's and Oleg's comments)
- Use 'same_ns' flag to drop SIGKILL/SIGSTOP to cinit from same
namespace


Limitations/side-effects of current design

- Container-init is immune to suicide - kill(getpid(), SIGKILL) is
ignored. Use exit() :-)


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